1. Field of the Invention
The present invention relates to an indoor mounting water heater in which combustion exhaust gas is discharged to the outdoors via an exhaust duct.
2. Description of the Related Art
Conventionally, there has been known a water heater comprising a combustion housing having an exhaust opening at the upper end thereof, a burner unit which is stored in the lower part of the combustion housing and configured by a plurality of burner groups, a heat exchanger for hot water supply which is stored in the upper part of the combustion housing, an air supply fan for supplying combustion air to the burner unit via an air supply chamber provided on the lower side of the arrangement section of burner unit in the combustion housing, and a capacity changeover mechanism for changing over the combustion capacity by changing the number of burner groups to be burned (for example, refer to Japanese Patent Laid-Open No. 11-294761).
In the case where the above-described water heater is used indoors by connecting the exhaust opening of the water heater to an exhaust duct already provided in a house or the like, if the gastightness of the exhaust duct is low, combustion exhaust gas may leak from an intermediate portion of the exhaust duct. If the exhaust duct is replaced with a high gastight duct, there arises no problem. However, the replacement of exhaust duct requires large-scale construction work, which results in an increase in cost. Therefore, to prevent the combustion exhaust gas from leaking even if the existing low gastight exhaust duct is used, it is necessary to increase the temperature of combustion exhaust gas of the water heater to make the draft force strong.
If the combustion capacity is decreased by the capacity changeover mechanism, some of the air supplied from the air supply fan passes through a non-burning burner group and is mixed with the combustion exhaust gas of a burning burner group in the upper part of the combustion housing, by which the temperature of combustion exhaust gas is decreased. Therefore, to maintain a strong draft force by increasing the temperature of combustion exhaust gas, the combustion capacity must always be kept in a high state by the capacity changeover mechanism. However, there arises a problem in that hot water is difficult to be supplied at a low set temperature.
Also, in the conventional water heater, if the combustion capacity is decreased by the capacity changeover mechanism, some of the air supplied from the air supply fan passes through a non-burning burner group and absorbs heat from the heat exchanger, which presents a problem of decreased heat efficiency.